The study conducted under this contract will evaluate the carcinogenic potential of theophylline on F344 rats and B6C3F1 mice. Groups of 50 male and female F344 rats will receive theophylline by gavage in corn oil at doses of 7.5, 25, or 75 mg/kg and groups of 50 male or female mice will receive doses of 15, 50, or 150 mg/kg, 5 days per week for 103 weeks. At the end of the study survivors will be necropsied, all tissue collected and processed for histopathology and slides evaluated.